1. Field of the Invention
The present invention relates to a transmission apparatus and method and, more particularly, to a transmission apparatus and method suitable for video or audio data transmission.
2. Description of the Related Art
There is a transmission apparatus which transmits data received from an input apparatus at a predetermined period as continuous frames having a predetermined structure. Such a transmission apparatus is useful for transmission of, for example, video or audio streaming data that requires punctual output. However, fluctuations in the data input period or deviations in the operation clock frequency between the input apparatus and the transmission apparatus generate a difference between the data input rate and the transmission rate. This leads to data loss or frame discontinuity, that is, an impediment to high-quality transmission of video or audio streaming data.
As a general measure to prevent this, a buffer memory is used to absorb the rate difference and level out the transmission rate. However, when data is input for a long time without interruption, uneven fluctuations in the data input period or deviations in the operation clock frequency between the input apparatus and the transmission apparatus cause overflow or underflow of the buffer memory.
This problem can be solved by using a technique disclosed in Japanese Patent Laid-Open No. 62-072251, which changes the length of a preamble included in a frame in accordance with the amount of data stored in the buffer memory.
However, the technique of Japanese Patent Laid-Open No. 62-072251 requires a buffer memory with a considerable capacity to execute processing according to the amount of stored data. This still causes a transmission delay.
Additionally, Japanese Patent Laid-Open No. 62-072251 discloses no operation for external device switching. In the arrangement of Japanese Patent Laid-Open No. 62-072251, the amount of data stored in the buffer memory may instantaneously increase or decrease due to a change of the data input timing upon switching the external device. This causes overflow or underflow of the buffer memory. To prevent this, it is necessary to prepare a buffer memory whose capacity is large enough to prevent overflow even when switching the external device, and perform the operation while storing, in the buffer memory, a sufficient amount of data to prevent underflow.
In this case, since the operation is executed while storing a considerable amount of data in the buffer memory, an unnecessary transmission delay occurs.